I use IQ mainly in Terrorist Hunt, but not much in multiplayer. My favorite is Kapkan for defense (people are completely oblivious to those damn tripmines, it's hilarious), and Thermite for attack (if you think you're safe behind that reinforced wall, think again). I still think every operative in the game has the potential to be the "best" operative - it all depends on who is using them. I like to play as a different operator every game to shake things up, and to familiarize myself with each of them.